We work in the light-front formalism and discuss photon Wigner functions
based on Ref. [1]}, [2]}. The gauge invariant
TMD correlation function of EM fields [3]} for
an unpolarized nucleus moving in \(+z\)  direction is defined as
\(\mathcal {W}^{\mu \nu ;\rho \sigma }(P_{1},p_{1})=\int \frac{d^{4}\xi }{(2\pi )^{4}}e^{ip_{1}\cdot \xi }\left\langle P_{1}\right|F^{\mu \nu }(0)F^{\rho \sigma }(\xi )\left|P_{1}\right\rangle .\) 
